数组下标'int [5][float]'的类型无效

这是我的全部代码（在C++中），以备不时之需。我用的是Eclipse开普勒。

//Tutorials from www.cplusplus.com/doc/tutorial combined
#include <iostream>
using namespace std;
//tutorial #1.1 Function with type -- Addition
int addition (int a, int b)
{
  int r;
  r=a+b;
  return (r);
}
//tutorial #1.2 Function with type -- Subtraction
int subtraction (int a, int b){
    int r;
    r=a-b;
    return (r);
}
//tutorial #1.3 Function with NO type
void printmessage (void)
{
  cout << "It's good to be the king." << 'n';
}
//tutorial #2.1 Arguments passed by reference
void duplicate (int& a, int& b, int& c)
{
  a*=2;
  b*=2;
  c*=2;
}
//tutorial #2.2 and 2.5 Arguments passed by value and Declaration of preferred inline function
inline void prevnext (int x, int& prev, int& next)
{
  prev = x-1;
  next = x+1;
}
//tutorial #2.3 Default values in parameters (functions)
int divide (int a, int b=2)
{
  int r;
  r=a/b;
  return (r);
}
//tutorial #2.4.1 Overloaded functions -- int cannot handle decimals
int operate (int a, int b)
{
  return (a*b);
}
//tutorial #2.4.2 Overloaded functions -- float is used with decimals
float operate (float a, float b)
{
  return (a/b);
}
//tutorial #2.6 Recursivity -- factorial calculator
long factorial (long a)
{
  if (a > 1)
   return (a * factorial (a-1));
  else
   return (1);
}
//tutorial #2.7 Declaring function prototypes
void odd (int a); //declared
void even (int a); //declared
void odd (int a) //defined
{
  if ((a%2)!=0) cout << "Number is odd.n";
  else even (a);
}
void even (int a) //defined
{
  if ((a%2)==0) cout << "Number is even.n";
  else odd (a);
}
//tutorial #3.1 Initializing and accessing the values of an array
float billy [] = {16, 2, 77, 40, 12071};
float k, result=0;
int main ()
{
    cout << "Name: My Name n";
    cout << "Homework: 1";
    cout << endl;
    cout << endl;
  int x=8, y=1, z, aa;
  float n=3.0,m=9.0;
  long number;
  z = addition (4,2);
  aa = subtraction (6,3);
    cout << "The sum is " << z << 'n';
    cout << "The subtraction is " << aa << 'n';
    cout << "The second subtraction is " << subtraction(x,y) << 'n';
  z = addition (x,y) - 3;
    cout << "The new sum is " << z << 'n';
  printmessage();
  duplicate (x,y,z);
    cout << "x=" << x << ", y=" << y << ", z=" << z << 'n';
  prevnext (x, y, z);
    cout << "Previous=" << y << ", Next=" << z;
    cout << endl;
    cout << "Division = " << divide (14);
    cout << endl;
    cout << "Second Division = " << divide (10,8);
    cout << endl;
    cout << "Operate Multiply: " << operate(x,y);
    cout << 'n';
    cout << "Operate Divide: " << operate(n,m);
    cout << endl;
    cout << "Please type a number: ";
      cin >> number;
      cout << number << "! = " << factorial (number);        
      cout << endl;
    int i;
      do {
        cout << "Type a number (type 0 to exit): ";
        cin >> i;
        odd (i);
      } while (i!=0);
      for ( k=0 ; k<5 ; k++ )
       {
          result += billy[k];
       }
       cout << result;
  return 0;
}

这是我编写的代码中主要存在问题的部分。更具体地说，结果+=billy[k]行抛出错误：

      for ( k=0 ; k<5 ; k++ )
       {
          result += billy[k];
       }
       cout << result;

这对应于：

//tutorial #3.1 Initializing and accessing the values of an array
float billy [] = {16, 2, 77, 40, 12071};
float k, result=0;

我需要帮助修复这段代码，但最重要的是，我想知道为什么我的代码会给我这个错误，以及我如何让它变得更好（在初级层面上——显然，我在这里没有实现高级概念）。

编辑11:10PM我忘了把它切换回int。我最初有int，但由于错误，我想我会尝试float，看看它是否有区别。